_H. arborea_ can change colour to a great extent, mostly in adaptation to its immediate surroundings, but ill health and moulting may also influence it. The change is slow. The usual colour is green, brightest on bright, sunny hot days, dull when the sky is overcast, or when it is windy and showery. Day and night have no influence upon the colour-changes. The hue of the green agrees mostly with that of the foliage on which the frog happens to take its rest, for instance a field of Indian corn, birch-trees, or oak-trees. I once received a consignment from Saxony. When the box with moss was unpacked, they were of the dullest greenish-grey; they were put into a wired-off corner of the yard and were given the freshly cut branches of a lime-tree to sit upon. On the following morning I at first looked for most of the frogs in vain. The leaves had withered and all those frogs which sat upon the dark brown branches had put on a light brown garb, mottled with darker patches. Another specimen, one of several which were at liberty in a greenhouse, took to resting on the frame of the window-pane, in a corner where putty, glass, and discoloured white paint met; in the morning it was always of a mottled leaden colour, but during the nocturnal hunting it was green. In the winter, the window-corner being of course cold, the frog remained stationary for several months, but kept the leaden grey colour, until one day in the early spring it was mottled with green, and soon after it joined its green mates. Liebe observed a half grown tree-frog which he kept in Gera {192}during the winter in a glass with water-cress. While the temperature was near freezing the frog sat in the water, very lethargic, breathing perhaps once every quarter of an hour. Its colour was light green. When the water-cress was cut and removed, the frog darkened and became at last quite a discoloured grey. When the water-cress was put back, the creature reassumed the light yellowish-green colour, remaining in its lethargic condition until it became lively in the spring sunshine.